1. Field of the Invention
The invention relates in general to protective relay systems, and more specifically to new and improved signal quality monitoring apparatus useful for determining if a protective relay signal received over a communication channel is suitable for use.
2. Description of the Prior Art
Protective relay systems for protecting electrical power transmission lines must be reliable, operating correctly when needed, and avoiding unnecessary operation. When a protective relay decision function receives a protective relay signal, or signals, from a communication channel, it is of critical importance that the signal quality be of such a level that the desired line between dependability and security is maintained.
In general, communication channel problems can be classified as follows:
(1) Dead channel;
(2) Signal strength outside the normal limits due to malfunctions or improper calibration of channel equipment;
(3) Noisy channel due to failing equipment, signal mixups, EMI, and the like; and
(4) Invalid signals due to unannounced channel equipment servicing, testing and calibration.
It would thus be desirable to provide a new and improved signal quality monitor for communication signals, which reliably and economically monitors all important signal parameters, and provides a go, no-go type of response which indicates whether or not all monitored parameters meet the prescribed standards.